The present invention relates to optical waveguide sensors. In particular, it relates to extrinsic optical waveguide sensors having an interferometric region which is used to measure changes in the refractive index of a particular sample.
Extrinsic Fizeau interferometric fiber optic sensors have been used for dynamic monitoring of strain, temperature, or pressure in mechanical structures. These types of sensors employ a fiber that acts as a conduit for optical power transmission to and from the sensing element. A typical extrinsic Fizeau interferometer comprises a single-mode fiber, used as an input/output fiber, and a multimode fiber, used purely as a reflector, to form an air gap within a silica tube. The gap acts as a Fizeau cavity. The far end of the multimode fiber is shattered so the reflections from the far end do not add to the detector noise. The Fresnel reflection from the glass/air interface at the front of the air gap (reference reflection) and the reflection from the air/glass interface at the far end of the air gap (sensing reflection) interferes in the input/output fiber. Although multiple reflections occur within the air gap, the effect of the reflections subsequent to the ones mentioned above can be shown to be negligible. The two fibers are allowed to move in the silica tube, and changes in the air gap length cause changes in the phase difference between the reference reflection and the sensing reflection. The phase difference is observed as changes in intensity of the light monitored at the output arm of a fused biconical tapered coupler.
The problem with these types of sensors is that they are only capable of measuring changes in strain, temperature, or pressure in mechanical structures. These types of sensors measure physical path changes such as strain, temperature, and pressure and not optical path differences. In turn, it is not possible to perform macroscopic chemical procedures using these types of sensors. When one desires to measure the refractive indices of various chemical or biological samples in situ, these types of sensors are prohibitive because of their configuration. In particular, the use of a silica tube to form a Fizeau cavity prohibits the introduction of a liquid phase sample into the cavity.
Therefore, it is an object of the present invention to provide an extrinsic optical waveguide sensor which is capable of measuring the refractive index of a sample in situ.
Another object of the present invention is to provide an extrinsic optical waveguide sensor that employs a holder having a sample input region and at least one interferometric region.
Another object of the present invention is to provide a sensor that employs absolute measurement techniques as opposed to relying on an intensity-based signal.
The aforementioned and other objects were achieved by the extrinsic optical waveguide sensor of the present invention. The sensor comprises a holder having at least one sample input region and at least one interferometric region. At least one optical fiber is positioned in the holder. Each optical fiber has an endface serving as an input and an output. At least one reflector is positioned in the holder in an operable relationship to each optical fiber endface, thus defining each interferometric region.
As a further embodiment, a broadband light source is positioned in an operable relationship to the optical fiber at an end opposite from the optical fiber endface. A coupler is positioned between the broadband light source and the interferometric region. A detector is positioned in an operable relationship to the coupler.
The device of the present invention is used to make in situ measurements of the refractive index of a sample. Examples of the various samples include but are not limited to chemical and biological samples. The present invention offers the advantage of being able to measure optical path differences in a liquid phase sample, something that was previously not achievable with extrinsic optical fiber Fizeau interferometric sensors.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention will be obtained by means of instrumentalities in combinations particularly pointed out in the appended claims.